A multichannel access (MCA) system is a system in which different users share the use of plural channels, and is also used widely for business use. And the MCA system is also used in various kinds of data communications in addition to a radio communication for voice.
The number of users in the MCA system has been rapidly increased in recent years, and systems suitable for data transmission are also in great demand. Thus, specifications of a digital MCA system have come to be determined and turned to practical use.
In such a system as this, a multilevel quadrature amplitude modulation (QAM) is used and for example, a multilevel quadrature amplitude modulation is used which may have states of 16 levels by performing a 4-level amplitude modulation by means of the respective two carrier waves different in phase by 90 degrees from each other.
In a digital MCA system of STD-32 set as a standard of Research & Development Center for Radio Systems, Incorporated Foundation (RCR) for a modulation method of a digital MCA system, an M16QAM method is adopted and four subcarriers are assumed to be used.
Such a system as this is composed to perform a time division transmission by means of a time division multiple access (TDMA) method so that plural radio stations can use the same frequency.
Now, a multilevel QAM modulated signal can be represented by the following expressions; EQU I(t)=A cos .PHI.(t) (1), EQU Q(t)=A sin .PHI.(t) (2).
Namely, the multilevel QAM modulated signal is modulated so as to have information of phase and amplitude. Such a modulated wave signal having information of phase and amplification as this needs to have an amplification with excellent linear characteristics applied to it. In case of thinking much of the linear characteristics, a class-A amplification is desired to be performed, but has a problem that it is poor in efficiency.
On the other hand, a radio apparatus used in mobile communication needs an amplification high in efficiency so as to suppress its battery consumption. Therefore, a power amplification stage in a mobile radio apparatus or in a portable radio apparatus used in mobile communication performs a class-AB amplification rather than a class-A amplification.
Thus, a circuit called a linearity compensation circuit (linearizer) of Cartesian-loop method is used as a linear-characteristics compensation means for keeping a high accuracy by suppressing distortion in phase and amplitude liable to be caused by a class-AB amplification.
FIG. 9 is a structural diagram showing a radio apparatus performing a linearity compensation by means of a linearizer. In FIG. 9, digital data generated by a DSP 101 are converted by D/A converters 102 and 103 into analog data. The analog data are signals (I signal and Q signal) which have direct-current offset voltages and are of waveform oscillating around the offset voltages. The analog data are orthogonally modulated by a quadrature modulator 104. The quadrature modulator 104 receives a quadrature local oscillation signal from an OSC 105 and generates a transmission IF signal. And a transmission IF circuit 106 IF-amplifies the transmission IF signal having a frequency (IF frequency) of the OSC 105, and a frequency converter 107 converts the frequency of the IF-amplified signal into a desired frequency by means of a local oscillation signal from an OSC 108 to generate a transmission signal. And a power amplifying circuit 109 power-amplifies the transmission signal having the desired frequency by means of a class-AB amplification and the like.
At this time, an attenuator 201 attenuates a part of the transmission signal from the power amplifying circuit 109 and a linearizer IF circuit 203 IF-amplifies the transmission signal whose frequency has been converted to the IF frequency by the local oscillation signal from the OSC 108. And the signal of IF frequency is orthogonally demodulated by the quadrature local oscillation signal from the OSC 105 in a quadrature demodulator 204. The transmission signal orthogonally demodulated in this manner is supplied to the quadrature modulator 104 as a feedback signal and a negative feedback is performed inside the quadrature modulator 104. Such a negative feedback makes it possible to keep linear characteristics of high accuracy by suppressing distortion in phase and amplitude generated in the power amplifying circuit 109 and the like.
(1) In case of applying the above-mentioned negative feedback, a potential slippage in an offset voltage contained in a feedback signal becomes a problem. Namely, since a baseband bandwidth of a transmission signal includes the vicinity of its direct-current component, the offset voltage cannot be removed from the feedback signal. PA1 (2) Further, when performing the above-mentioned negative feedback, a phase difference between signals generated by the D/A converters 102 and 103 and a feedback signal generated by the quadrature demodulator 204 comes into question. Namely, a phase difference between the signals is caused by difference in length of physical paths and variation in circuit devices. PA1 (3) Further, since a multilevel QAM modulated signal needs to be amplified accurately in both of phase and amplitude, it has been also expected to materialize a radio apparatus capable of compensating linear characteristics of the power amplification stage by means of a negative feedback of a linearizer by automatic adjusting both of such an offset voltage and a phase difference as described above. PA1 (1) A first embodiment of the invention is a radio apparatus which compensates linear characteristics of a power amplification stage by feeding back to a quadrature modulator a signal component obtained by quadrature demodulation of a transmission signal detected from the power amplification stage at a transmission slot timing repeated at intervals of a specified time, said radio apparatus comprising an error voltage compensating means for making the respective offset voltages of signal components obtained by quadrature demodulation coincide with the respective offset voltages of signal components obtained by quadrature modulation by comparing the respective offset voltages of the signal components obtained by the quadrature demodulation during non-transmission periods with the respective offset voltages of the signal components obtained by the quadrature modulation. The non-transmission periods are periods other than transmission slot timing periods. PA1 (2) A second embodiment of the invention is a radio apparatus which compensates linear characteristics of a power amplification stage by feeding back to a quadrature modulator a signal component obtained by quadrature demodulation of a transmission signal detected from the power amplification stage at a transmission slot timing repeated at intervals of a specified time, said radio apparatus comprising a phase adjusting means for making a signal component obtained by quadrature demodulation of a transmission signal coincide in phase with a signal component obtained by quadrature modulation through adjusting in phase a local oscillation signal to be supplied to the quadrature demodulator. PA1 (3) A third embodiment of the invention is a radio apparatus which compensates linear characteristics of a power amplification stage by feeding back to a quadrature modulator a signal component obtained by quadrature demodulation of a transmission signal detected from the power amplification stage at a transmission slot timing repeated at intervals of a specified time, said radio apparatus comprising;
However, a slippage in direct-current potential (offset voltage) between signals generated by the D/A converters 102 and 103 and a feedback signal generated by the quadrature demodulator 204 is generated due to a little difference in characteristics of the respective circuits of the D/A converters, the quadrature modulator and the quadrature demodulator, variation of temperature and the like. The slippage in direct-current voltage is greatly amplified when applying a great amount of negative feedback, which may reversely deteriorate the linear characteristics of the power amplification stage.
Up to now, therefore, parameters of the various parts have been fixed by manually adjusting the offset voltage. However, since some error is caused by aging or variation of temperature, there is a problem that it is difficult to obtain stable characteristics. And there is also a problem that the adjusting operation by hand itself is complicated.
Although there is a technique for automatically adjusting a slippage of the offset voltage only in a quadrature modulator, there is not an apparatus for automatically adjusting a slippage in offset voltage all over the transmission and the negative feedback system as described above.
Therefore, it has been expected to materialize a radio apparatus capable of automatically adjusting a slippage in offset voltage between the transmission system and the negative feedback system in a radio apparatus using a linearizer as described above.
Such a phase difference is not preferable to applying of a negative feedback. Hence, it has been attempted to compensate the above-mentioned phase deference by adjusting in phase either one of local oscillation signals to be supplied to the quadrature modulator and the quadrature demodulator.
A method of adjusting a variable resistor of a phase shifter composed of an electrical circuit is thought as such a phase adjustment. In this case, up to now, parameters of the various parts have been fixed by manually adjusting the offset voltage. However, since some error is caused by aging or variation of temperature, there is a problem that it is difficult to obtain stable characteristics. And there is also a problem that the adjusting operation by hand itself is complicated.
Further, a method of adjusting physical paths in length or the like is thought as another phase adjusting method. Although such a method is not as susceptible to aging or variation of temperature as the variable resistor adjusting method, an apparatus where physical paths are adjusted has an increased size and high cost.
Furthermore, in case that the apparatus is used in a frequency different from the frequency in which a phase adjustment has been made, any of the phase adjusting methods has a problem that a further phase lead or phase lag is caused by variation of LC components of the circuit devices and variation in electrical path length. In case of changing a transmission frequency of a radio apparatus, linear characteristics of the power amplification stage may be reversely deteriorated by the negative feedback of the linearizer.
Therefore, it has been expected to materialize a radio apparatus capable of automatically adjusting a slippage in offset voltage between the transmission system and the negative feedback system in a radio apparatus using a linearizer as described above.